Originally Posted by saltysenior
again i bring up the question of these so called ''tests''......where were they run???? i'll run a boat in 10 feet of water and come up w/figures that will make a sister ship running in 90feet look like a dog...
Depth will not give a 30 knot boat any more speed from everything I've seen. Running a 65' sf at 30 knots in 10 feet of water or 300 feet of water, the speed will be virtually the same. You're thinking of a big heavy slow motoryacht like the 75' Hatteras I used to run that cruised at 16.5-17 knots. In that case the shallow water gave it a little more stern lift because it was not totally on plane. But at 30 knots the hull is totally on plane and is getting it's lift from the speed of the water running under the hull and not from the propellors or prop wash bouncing off the bottom giving it lift.
I do 10,000 NM's in deliveries a year, and always on different yachts. The majority of it ocean miles, but also a lot of inland miles too. I also run 150 different yachts a year of all types from 30-103'. I've done a lot of engine startup's on new boats, and several of the same type and engine type, both inland and in the ocean and the top speeds I've achieved are virtually identical. The only difference I've seen is that if it's rough on the ocean test, it will effect speed a bit, so will current to a smaller bit. In fact on a lot of 1 30 knot sportfish manufacturer I've done a lot of engine start ups on (over 100 boats over the years), the real world numbers will be 2 knots faster then the test numbers because we use NO trim tabs to keep every test consistent.
It's like running a boat in an area with a lot of current. For example hell's gate. If you run an 8knot displacement trawler, against the 8 knot current, you will have next to no headway. Maybe 1-2 knots. If you run a 30 knot boat against the 8 knot current, you'll still be doing 28 knots....... The faster the boat is, current and depth have less of an effect, mainly because less of the boat is in the water and less drag.